The present invention relates to a multi-directional communication system, such as a duplex system. Such a system may, for example, be used for data communication using modem links over a subscriber line, network, or other communication link. The invention is particularly suitable for implementation using conventional wire links, but the invention is equally applicable to other communication mediums, for example, wire-less links and optical fibre links.
A number of data transmission techniques have been developed with the aim of providing high speed communication over existing communication mediums, for example, a subscriber line. Examples include Integrated Services Digital Network (ISDN) and Asymmetric Digital Subscribers Line (ADSL). ISDN provides full duplex communication with the same capacity in either direction of communication. ADSL provides asymmetric capacities (for example, for applications such as Internet access in which more information is expected to flow in one direction than the other).
Reference is made to U.S. Pat. No. 5,668,802 which describes a high speed digital subscriber line employing discrete multiple tone (DMT) signals. DMT is an example of orthogonal frequency division multiplexing, in which the signal on the line consists of encoded multiple harmonic carriers, each carrier representing a xe2x80x9ccommunication channelxe2x80x9d. The carriers are modulated with a signal representing a time-varying data frame in a discrete frequency domain. The capacity of the line in either direction is fixed; a first group of the channels (e.g. the odd numbered channels) are used exclusively for communication in one direction, and a second group of the channels (e.g. the even numbered channels) are used exclusively for communication in the other direction.
In devising the present invention, it has been appreciated that the above techniques, which employ fixed data rates in either direction, can only use the communication system efficiently when the demand in each direction closely matches the respective data rate capacity fixed for that direction. If the demand in either direction does not match that provided by the communication link, the efficiency of communication may be significantly reduced, since redundant capacity in one direction is not usable to increase capacity in the other direction. Not only does this result in increased communication time (and the consequent increased expense of, for example, a subscriber line), it can also require the need for expensive large buffers to accommodate temporary buffering of data bottlenecking while awaiting transmission.
In contrast to the prior art technique discussed above, in which channels are permanently reserved for information communication in specific directions, one aspect of the present invention is to enable control of the relative capacities of first and second channels for a multidirectional communication system by controlling the relative number of sub-channels (from a communication band) allocated to each channel.
The term xe2x80x9cchannelxe2x80x9d is used herein in a broad sense to denote a stream of information being carried by one or more information carrying sub-channels representing the channel.
Preferably, the allocation is controllable dynamically, such that the relative capacities of the channels can be varied, for example, to suit temporal demand for communication capacity in each channel.
With this aspect of the invention, the efficiency of multidirection communication can be improved significantly by enabling redundant capacity in one channel to be used to increase the capacity of another channel when demand for capacity in that other channel is high. For example, in times of heavy capacity demand in the first channel, the capacity of the first channel relative to the second channel can be increased by allocating more sub-channels to the first channel than to the second. Likewise, in times of heavy demand in the second channel, the capacity of the second channel can be increased relative to the first by allocating (or re-allocating) a greater number of sub-channels to the second channel.
Preferably, the capacity is dependent on the number of sub-channels allocated to the channel, the capacity increasing with a greater number of available sub-channels.
The determination of capacity demand for each channel can be made in a number of ways, for example, by monitoring the whether the xe2x80x9clivexe2x80x9d traffic in each channel matches the current channel capacity, or by monitoring the size of input buffer files for data awaiting transmission to each channel.
Preferably, the communication system employs multi-carrier modulation. Preferably, each carrier corresponds to a sub-channel of the communication band.
More preferably, the system employs orthogonal frequency division modulation (OFDM) in which the carriers (also referred to as sub-carriers) are separated in frequency by the baud rate of the modulation (or by a multiple thereof) such that the sub-carriers are theoretically independent. Most preferably, the system employs coded orthogonal frequency division multiplexing (COFDM) in which one or more redundant coding techniques are used to enable at least limited error correction.
In a closely related aspect, the invention provides apparatus for use in a multidirectional communication system for enabling control of the relative capacities of first and second communication channels by controlling the relative number of sub-channels allocated for each channel.
In a further closely related aspect, the invention provides a method of operating a multi-directional communication system, the method comprising controlling the relative capacities of first and second channels in the system by controlling the relative number of sub-channels allocated to each channel.
In a yet further aspect, the invention provides a multidirection communication system including means for controlling the relative capacities of first and second channels thereof in response to the demand for capacity for at least one of the channels. Preferably, the control means is operable to vary the relative capacities in response to changes in the demand.
Preferably, the relative capacities are controlled in response to the demand for capacity for each channel.
Preferably, the system comprises means for determining the demand for capacity for each channel.
The capacity of a channel can be controlled by varying the bandwidth of the channel, for example, by varying the number of sub-channels allocated to the channel.
In a closely related aspect, the invention provides apparatus for use in a multidirectional communication system, the apparatus comprising means for controlling the relative capacities of first and second channels of the system in response to the demand for capacity for at least one of the channels.
In yet further closely related aspect, the invention provides a method of operating a multi-directional communication system, the method comprising controlling the relative capacities of first and second channels of the system in response to the demand for capacity for at least one of the channels.
In a further aspect, the invention relates to an OFDM system, and to synchronisation techniques for improving operation.
Referring again to U.S. Pat. No. 5,668,802, special time synchronisation schemes are described in order to permit the transmitters at the opposite ends of the subscriber line to transmit a data frame according to a common xe2x80x9clinexe2x80x9d timing. A first scheme involves adding a post-cursor to a data frame to repeat data from the beginning of the frame; a second scheme involves a master station commanding the slave to introduce xe2x80x9cslipxe2x80x9d between the reception and transmission frames (at the slave end).
In developing this aspect of the present invention, it was appreciated that interference between OFDM transmissions in each direction can be reduced by synchronising each of the OFDM transmitters/receivers using a common source.
In accordance with this aspect, the invention provides an OFDM communication system comprising:
a first OFDM device at a first station;
a second OFDM device at a second station;
means at the first station for sending a frequency synchronisation signal from the first station to the second station; and
means at the second station for receiving the frequency synchronisation signal and for frequency synchronising the second OFDM device using the frequency synchronisation signal.
Each OFDM device may be a transceiver having a receive and a transmit function, or a circuit having only a receive function or only a transmit function.
Preferably, the synchronisation signal is derived from the frequency used by the first OFDM device, or the first OFDM device is also frequency synchronised to the synchronisation signal.
The frequency synchronisation signal may, for example, comprise a burst at a predetermined frequency, or a synchronisation code or symbol from which synchronisation information can be derived.
The frequency synchronisation signal may also represent timing information usable at the second station to time synchronise the second station to the first station. For example, the timing information may comprise the leading, or trailing, edge of the burst signal, and define a precise reference point in time relative to transmission of OFDM xe2x80x9csymbolsxe2x80x9d. In the case of a synchronisation code, synchronisation can be monitored by detecting the energy from the FFT circuit; the energy will be a maximum when receiver is correctly time synchronised.
With this aspect of the invention, it is possible to reduce the effects of design and operating tolerances which might normally cause slight differences in the timing or frequency associated with each independent OFDM device. Such small variations would normally tend to reduce the true orthogonality of the different sub-carriers generated or used by the different OFDM devices, and hence produce a degree of interference between the sub-carriers. By improving the overall synchronisation, a high degree of orthogonality can be attained, leading to improved isolation between the transmission and reception sub-carriers. Such a signal may be suitable for transmission over long transmission distances.
In a closely related aspect, the invention provides apparatus for use in a communication system, the apparatus comprising a first OFDM device for communicating with a second remote OFDM device at a remote station, and means for generating a frequency synchronisation signal for transmission to the remote station to facilitate frequency synchronisation of the second OFDM device to the first OFDM device.
In a closely related aspect, the invention provides apparatus for use in a communication system, the apparatus comprising a first OFDM device for communicating with a second remote OFDM device at a remote station, means for receiving from the remote station a frequency synchronisation signal indicative of the frequency used by the second OFDM device, and means for synchronising a frequency of the first OFDM device to the received synchronisation signal.
In a closely related aspect, the invention provides a method of operating a communication system comprising a first OFDM device at a first station and a second OFDM device at a second station, the method comprising:
generating at the first station, a frequency synchronisation signal;
transmitting the frequency synchronisation signal to the second station; and
using the received frequency synchronisation signal at the second station to frequency synchronise the second OFDM device.
In a further closely related aspect, the invention provides a method of operating a first OFDM station for communication with a second remote OFDM station, the method comprising:
generating at the first station, a frequency synchronisation signal representative of a reference frequency used in the first OFDM station; and
transmitting the frequency synchronisation signal from the first station for communication to the second station to enable frequency synchronisation of the circuitry in the OFDM station relative to the OFDM circuitry at the first station.
In a further closely related aspect, the invention provides a method of operating a first OFDM station for communication with a second remote OFDM station, the method comprising:
receiving at the first station a frequency synchronisation signal representative of a reference frequency used with the second OFDM station; and
using the received frequency synchronisation signal to synchronise the first OFDM station at a corresponding operation frequency.
In a yet further aspect, the invention relates to communication over a network consisting of three or more stations.
In accordance with this aspect, the invention provides an OFDM communication network comprising first, second and third communication nodes, wherein the OFDM sub-channels are distributed for simultaneous communication from the first node to the second node, and from the third node to the first or second node.
In developing this aspect of the invention, it was appreciated that multiplexed communication may be achieved by allocating the OFDM sub-channels to allow simultaneous communication between first, second and third nodes in a network. The principles of two-way communication can thus be expanded to provide communication channels between more than two nodes, stations, or modems.
The sub-channel allocations may be predetermined such that the capacities are fixed, and are not variable in use. Alternatively, the controllable sub-channel allocation techniques discussed above may be employed to provide variable capacities to suit demand.
The frequency synchronisation techniques discussed above may also be employed to improve synchronisation between the different OFDM circuits at each node. Generally one node will be designated as a master, and all other nodes will be designated as slaves to follow the synchronisation signal generated by the master.